It is known that gases are characterized in particular by molecular or atomic absorption lines at wavelengths that are determined very accurately and that it is possible to detect the presence of a gas in the atmosphere by identifying one or more absorption lines.
A "lidar" type system can be used for this purpose, the system comprising a laser emitting pulses towards the observed volume at a wavelength which corresponds to an absorption line of the looked-for gas. The signals backscattered by the atmosphere are detected, and the presence or absence of the gas in the atmosphere is deduced therefrom.
That known system suffers from certain drawbacks, including relatively high cost, weight, and bulk, with uses thereof being limited to the wavelengths of the laser pulses, and with the dangers associated with emitting high-energy pulses (danger for the eye, risk of explosion in certain environments, e.g. of petroleum gas or of chemical gas).
Proposals have also been made to detect the presence of a gas in the atmosphere by radiometric or spectroscopic techniques, enabling a spectral distribution of energy received or transmitted over a band of wavelengths to be obtained, with such techniques being based on the fact that a gas which absorbs energy at a certain wavelength also re-emits energy at said wavelength. However, the sensitivity of such techniques is highly variable. To obtain high sensitivity, it is necessary to control the spectral emittance of the source and to dispose the emitter and the receiver on opposite sides of the observed volume, or else when the emitter and the receiver are on the same side of the observed volume, to dispose a reflector on the other side of said volume, thereby requiring two points of access to said volume, and also fixing the measurement optical path. In other cases where the characteristics of the emitter, and in particular its temperature, are not under control, the results obtained depend on the difference between the temperature of the gas and the background temperature, i.e. the temperature to be found behind the gas and which serves as an emitter, with sensitivity being zero when said difference is zero.
Patent GB 2 176 889 proposes a system for detecting a gas in a volume to be monitored, on the basis of the ratio of the radiant fluxes from said volume in two wavelength bands. One of which corresponds substantially to an absorption line of the gas and the other of which essentially comprises wavelengths in which the gas does not have an absorption peak. A drawback of that system is that it operates only if the gas to be detected is considerably colder than its environment.
An object of the invention is to provide a method and apparatus for passive detection of a gas present in an observed volume, but without being subjected to the above drawbacks.
Another object of the invention is to provide a method and apparatus of this type enabling a gas present in an observed volume to be detected remotely from a single point of access to said volume.
Another object of the invention is to provide a method and apparatus of this type enabling a gas present in an observed volume to be detected quickly in real time.
Another object of the invention is to provide a method and apparatus of this type enabling the atmospheric environment of an area such as an industrial site, e.g. an oil refinery or a chemical factory, to be monitored remotely.